L.C.P. da Silva

Comparative analysis between synchronous and induction machines for distributed generation applications

This paper presents a detailed comparative analysis between synchronous and induction machines for distributed generation applications. The impacts of these generators on the distribution network performance are determined and compared by using computational simulations. The technical factors analyzed are steady-state voltage profile, electrical power losses, voltage stability, transient stability, voltage sags during unbalanced faults, and short-circuit currents. The results showed that the best technical choice depends on the network characteristics, i.e., the main factors that may limit the penetration level of distributed generation.

MW and MVar management on supply and demand side for meeting voltage stability margin criteria

This paper presents a methodology to improve the power system economical dispatch from a voltage stability margin perspective. The time horizon under discussion is the short-term operation planning. The proposed method is based on active/reactive power re-dispatch for normal operation, and also minimum load shedding strategies in case of critical contingencies. The actions are taken in the direction provided by modal participation factors computed for generator and load buses. The generators with negative impact on system margin, which are indicated by the modal index, are penalized with high costs on the objective function of the optimal power flow program used to run the re-dispatch process. Results of this work show a decrease on system losses and significant increase on voltage stability margin as well as on system reactive reserves. In addition, this work presents a study considering critical contingencies, for which is proposed an optimal load shedding strategy also based on modal participation factors to identify the most adequate buses for load shedding purposes. Finally, the proposed methodology is applied considering a typical hour-to-hour daily load curve, and the method presented very good performance since it considerably increases voltage stability margin for the insecure intervals.

Valuation of Dynamic Reactive Power Support Services for Transmission Access

Competitive procurement of reactive power support services is rapidly becoming a reality for deregulated electricity markets. It has resulted in a great need to quantify the value of reactive power support from var sources. This article presents research results on the development of new concepts and schemes for equitable reactive power support valuation. Performance characteristics of the proposed method are determined and practical application issues are addressed. The validity of the method is verified through sensitivity studies. This work emphasizes that the valuation of reactive power support services should be based on their contributions to system security and stability. The dynamic var is the primary concern for the reactive power valuation problem.

Continuation fast decoupled power flow with secant predictor

The conventional Newton and fast decoupled power flow methods are considered inadequate for obtaining the maximum loading point of power systems due to ill-conditioning problems at and near this critical point. At this point, the Jacobian matrix of the Newton method becomes singular. In addition, it is widely accepted that the P-V and Q-/spl theta/ decoupling assumptions made for the fast decoupled power flow formulation no longer hold. However, in this paper, a new fast decoupled power flow is presented that becomes adequate for the computation of the maximum loading point by simply using the reactive power injection of a selected PV bus as a continuation parameter. Besides, fast decoupled methods using V and /spl theta/ as parameters and a secant predictor are also presented. These new versions are compared to each other with the purpose of pointing out their features, as well as the influence of reactive power and transformer tap limits. The results obtained for the IEEE systems (14 and 118 buses) show that the characteristics of the conventional method are enhanced and the region of convergence around the singular solution is enlarged.

Dynamic VAr sources scheduling for improving voltage stability margin

This paper presents a methodology to be added at the power system dispatch problem in order to evaluate and improve voltage stability margin by optimizing generators and synchronous condensers reactive power injection. The main objective is to reschedule the reactive injection of the machines, from a perspective of improving voltage stability margin, without impact on the active economical dispatch.

Small-disturbance voltage stability of distribution systems with induction generators

This letter presents an investigation about the small-disturbance voltage stability of distribution systems with induction generators by using time-domain nonlinear dynamic simulations. Results show that the presence of induction generators may decrease the system voltage stability margin. It was verified that at the maximum loading point, if the system loading is increased even more, then the induction generator accelerates to a high speed, becoming unstable and leading the system to a voltage collapse.

Real power losses reduction and loading margin improvement via continuation method

This letter presents an alternative approach for reducing the total real power losses by using a continuation method. Results for two simple test systems and for the IEEE 57-bus system show that this procedure results in larger voltage stability margin. Besides, the reduction of real power losses obtained with this procedure leads to significant money savings and, simultaneously, to voltage profile improvement. Comparison between the solution of an optimal power flow and the proposed method shows that the latter can provide near optimal results and so, it can be a reasonable alternative to power system voltage stability enhancement.

Competitive procurement of dynamic reactive power support service for transmission access

Competitive procurement of reactive power support services is rapidly becoming a reality for deregulated electricity markets. This has resulted in a great need to quantify the value and to compensate the service of reactive power support. This paper presents preliminary research results on the development of concepts and schemes for equitable reactive power support valuation. The work emphasizes that the valuation of reactive power support services must be based on their contributions to system security and stability. The dynamic VAr support is of much greater importance in the value assessment. The paper uses a simple system to demonstrate and define the problem, and introduces the concept of value curves. A solution method is proposed along with preliminary case study results.

Investigation of the relationship between ill-conditioned power flow and voltage collapse

Ill-conditioned power flow problems have been widely investigated and discussed in the literature. Typical approaches are to develop enhanced solution algorithms when a power flow case is found to diverge using the conventional Newton method. It is known that a genuine ill-conditioned problem is caused by the presence of a large condition number in the power flow Jacobian matrix. Since a large condition number is associated with small singular values and the voltage collapse is related to small eigenvalues, we therefore speculate that an ill-conditioned power flow problem is actually a voltage collapse problem. The objective of this letter is to demonstrate the relationship between power flow ill conditioning and voltage instability. Case studies of five classical ill-conditioned cases have confirmed that the ill conditioning of power flow indeed occurs-and only occurs-at the voltage collapse point.

MVAr management on the pre-dispatch problem for improving voltage stability margin

This paper presents a methodology that includes at the power system pre-dispatch problem the evaluation and improvement of voltage stability margin by optimizing generators and synchronous condensers reactive power injections. From modal participation. factors it is defined penalty indices for all generators, which are added to the optimal power flow objective function. The purpose is to obtain the most adequate reactive power injection for each generator or synchronous condenser, from a perspective of maximizing voltage stability margins. Preliminary results presented in this paper, obtained for the New England test system of 39 buses and 10 generators, show that the proposed methodology leads to significant voltage stability margin improvement for all the critical time intervals of the day. A clear advantage of the proposed methodology is that the optimal solution for generators active injection is kept unchanged. It means no impact on generators energetic targets, and no impact on the total generation cost. Hence, voltage stability margin is improved by just managing the reactive power dispatch.